This invention relates generally to devices which check the level of a liquid in a tank by utilizing a partially submerged electrical resistance probe. More particularly this invention relates to a digital sample and hold circuit which memorizes the initial voltage across the electrical resistance probe.
Electrical resistance or hot-wire fluid level detection systems depend on the detection of probe resistance increase resulting from the application of a test current. The hot wire is mounted in a probe which in turn is positioned in the fluid to be monitored. The positive temperature coefficient characteristic of the hot wire produces a probe resistance increase when the test current is applied. The magnitude of this increase is proportional to the percentage of the hot wire element which is not immersed in fluid.
The sequence of electrical events which describe the complete fluid level sensing system are the application of current to the probe during the very short test interval, memorization of the initial voltage developed across the probe, measurement of the amplitude of the resistance change, comparison of the resistance change to a preselected threshold and the triggering of a warming signal when the fluid level is below the level corresponding to the preselected threshold.
In order to prevent the system from responding to resistance changes attrributable solely to the temperature of the probe environment, it is necessary to measure and memorize the initial resistance of the probe at the moment current is initially applied and to compare that initial resistance to the final resistance at the end of the test interval.
The warning threshold is selected to be between the energized probe's outputs representing the free air condition and the completely submersed condition.
Previous known designs have depended upon analog sample and hold circuits to memorize the initial probe resistance. Analog sample and hold circuits employ capacitors to retain a voltage proportional to the initial condition to be memorized. If the charge on the capacitor leaks off during the hold period, a memory error will occur. Conventional means to minimize the effects of this leakage effect are not cost effective.
Accordingly, it is an object of this invention to employ a digital sample and hold circuit to permit retention of any desired initial condition without the introduction of memory errors due to capacitor leakage.
Other objects and advantages of this invention will be made apparent as the description progresses.